A self-tightening drill chuck as described in German patent 3,432,918 of the instant inventor has a chuck body centered on and rotatable about an axis, an adjustment body rotatable but axially not displace able on the chuck body, and a plurality of jaws radially displaceable on one of the bodies and each formed with a row of teeth. The other body is formed with a screwthread meshing with the jaw teeth so that rotation of the one body relative to the other body in a tightening direction moves the jaws together and rotation in the opposite direction moves the jaws apart. A locking ring is axially displaceable on the chuck body between a freeing position and a locking position and a tightening spring connected between the locking ring and chuck body urges the locking ring rotationally on the body in the tightening direction. The locking ring is provided with a radially inwardly projecting pin that engages in a radially outwardly open recess of the spindle on which the chuck body is mounted to limit the angular movement of the locking element on the chuck body. Formations couple the locking ring to the adjustment body in the locking position and decouple these parts in the freeing position. A tightening spring urges the locking ring in the tightening direction and a locking spring urges the locking ring into the locking position.
To tighten such a chuck the locking ring is held in one hand and the adjustment body in the other and the adjustment ring is twisted in the tightening direction on the locking ring. Once the jaws engage the chuck, all the relative torque between the adjustment and locking rings is therefore applied at the end of the coupling pin to the drill spindle. This location is relatively close to the axis and relatively small, so that there is a substantial likelihood of material deformation or breakage.